This might be an issue with your browser itself, and not necessarily malware.
Now, let's reset your Firefox browser first, as follows:
First off, please click on the orange Firefox tab at the top-left in Firefox, then click Help > Restart with Add-ons Disabled > Ok. Now, Firefox will close and a dialog box will come up - check all of the options EXCEPT for the one to delete your bookmarks, then click "Make changes and restart".
Now, Firefox will come up the default way.
Done that, to make sure that a virus/malware isn't causing this issue either, download the following freeware (don't buy anything) security program, Malwarebytes' Anti-Malware
(click the blue 'Download now' button next to the down arrow icon)
Download, run and install the same, then carry out a quick system scan using the program, and remove the security threats detected, if any.
This should do it.